The present invention relates to a servo control apparatus, and more specifically, to a servo control apparatus for controlling a servo motor used to drive a machine tool and the like.
FIG. 14 is a block diagram for showing a conventional servo control apparatus. In this drawing, reference numeral 1 indicates a position command producing unit, reference numeral 2 represents a position control unit, reference numeral 3 indicates a speed control unit, and reference numeral 4 is a mechanical resonance suppressing filter, which is constructed of, for instance, a notch filter and the like, and is provided so as to remove resonance and vibrations caused by a characteristic frequency of a machine system and the like, and reference numeral 5 shows a current control unit, reference numeral 6 indicates a current drive means constituted by, for example, a power amplifying circuit and the like. Also, reference numeral 7 denotes a servo motor for driving the machine system, reference numeral 8 represents an encoder for detecting a rotary position of the servo motor 7, and reference numeral 9 shows a differentiating means for differentiating a position detection signal outputted from the encoder 8. It should be noted that a detection means for detecting the motor speed is composed of the encoder 8 and the differentiating means 9.
Reference numeral 10 shows a position command signal outputted from the position command producing unit 1, reference numeral 11 represents a position feedback signal indicative of a rotary position of the servo motor 7, which is outputted from the encoder 8, and reference numeral 12 is a speed command signal outputted from the position control unit 2, reference numeral 13 indicates a speed detection signal (speed feedback signal) outputted from the differentiating means 9, reference numeral 14 shows a speed deviation signal equal to a difference between the speed command signal 12 and the speed detection signal 13, reference numeral 15 represents a current command signal outputted from the speed control unit 3, reference numeral 16 denotes a filter output current command signal which is again produced via the mechanical resonance suppressing filter 4, and reference numeral 17 indicates a current feedback signal indicative of a current flowing through the servo motor 7.
This servo control apparatus is so arranged as to control the servo motor 7 in such a manner that the position feedback signal 11 indicative of the rotary position of the servo motor 7 which is detected by the encoder 8 may follow the position command signal 10 outputted from the position command producing unit 1. Also, in order to perform this operation in a high speed and under stable condition, the position control unit 2 produces the speed command signal 12 based upon the deviation signal between the position command signal 10 and the position feedback signal 11. Furthermore, the speed control unit 3 outputs the current command signal 15 to the servo motor 7 in such a manner that the speed feedback signal 13 which is produced by the differentiating means 9 based upon the position feedback signal 11 may follow the speed command signal 12.
As previously explained, reference numeral 16 shows a filter output current command signal which is outputted from the mechanical resonance suppressing filter 4. Both the current control unit 5 and the power amplifier 6 controls the current which is supplied to the servo motor 7 in order that the current feedback signal 17 Which indicates a value of a current flowing through the servo motor 7 may follow the current command signal 16. Also, reference 18 indicates a machine system driven by the servo motor 7, and reaction force 19 produced by this machine system may give an effect to the power of the servo motor 7.
In this case, in order to increase the following characteristic with respect to the command of the servo motor 7, while either the resonance characteristic of the position loop which is defined from the position command signal 10 to the position feedback signal 11 or the response characteristic of the speed loop which is defined from the speed command signal 12 to the speed feedback signal 13 are increased, the mechanical resonance and the vibrations are produced due to the characteristic frequencies existing in the machine system 18 driven by the servo motor 7. As a result, not only the machine system 18 is operated under unstable condition, but also the control loop itself of the servo motor 7 is brought into the unstable condition.
In the above-described conventional servo control apparatus, while the mechanical resonance suppression filter 4 is inserted between the current command signals 15 and 16 so as to reduce the mechanical resonance and the vibrations, in the case where the characteristic frequency of the mechine system 18 is low and the frequencies of the mechanical resonance and of the vibrations are low, the resonance eliminating frequency of the mechanical resonance suppressing filter 4 must be set to the low frequency. As a result, when this setting frequency of the mechanical resonance suppressing filter 4 approaches the response range of the speed loop, the adverse influence caused by the phase delay in the servo control loop, in particular, defined from the speed command signal 12 to the speed feedback signal 13 is increased, so that the phase margin of the speed loop corresponding to the closed loop defined from the speed command signal 12 to the speed feedback signal 13 is lost and thus, the control system is brought into the unstable condition. This may cause a problem in that enhancement of the performance is hindered while the response characteristic of the servo control system is increased and also the following characteristic of the position feedback signal 11 to the position command signal 10, namely the major purpose of the servo system, is increased.
Also, as the mechanical resonance suppressing filter 4, there are many cases that notch filters capable of removing only certain fixed frequency components are used. However, since there are many cases in which a plurality of characteristic frequencies are present in the machine system 18, it is practically difficult to remove all of the mechanical resonance by employing such notch filters. To the contrary, one solution may be conceived. That is, while a low-pass filter is employed as the mechanical resonance suppressing filter 4, the gain higher than, or equal to a certain specific frequency is reduced. In this case, since the phase delay is commenced from the very low frequency range with respect to the setting frequency, there are such problems that the adverse influence is easily given also to the control range of the speed loop, and the phase margin of the speed loop with respect to the notch filter can be hardly obtained.